I'd say take the date in advance 2-3 months (or more) depending on how much time you think you'll need to study. This would give you a dealine to work towards, and of course you can always reschedule for a small fee. It is possible to get a time within a week; a couple of my pre-med friends did with the MCAT, but if you are unlucky you might have to wait longer than you'd like to.